Chapter 5 Methods

1. Chapter 5 Methods

2. Objectives • To declare methods, invoke methods, and pass arguments to a method (§5.2-5.4). • To use method overloading and know ambiguous overloading (§5.5). • To determine the scope of local variables (§5.6). • To learn the concept of method abstraction (§5.7). • To know how to use the methods in the Math class (§5.8). • To design and implement methods using stepwise refinement (§5.10). • To group classes into packages (§5.11 Optional).

3. Introducing Methods A method is a collection of statements that are grouped together to perform an operation.

4. public class MaxExample {public static void main(String[] args){ int x, y, maxVal; String strNum; strNum = JOptionPane.showInputDialog( "Enter an integer" ); x = Integer.parseInt( strNum ); strNum = JOptionPane.showInputDialog( "Enter another int" ); y = Integer.parseInt( strNum ); maxVal = max( x, y ); JOptionPane.showMessageDialog(null, "The max is " + maxVal );} public static int max( int num1, int num2 ) { return num1 > num2 ? num1 : num2; } }

5. Introducing Methods, cont. • Method signature is the combination of the method name and the parameter list. • The variables defined in the method header are known as formal parameters. • When a method is invoked, you pass a value to the parameter. This value is referred to as actual parameter or argument.

6. Introducing Methods, cont. • A method may return a value. The returnValueType is the data type of the value the method returns. If the method does not return a value, the returnValueType is the keyword void. For example, the returnValueType in the main method is void.

7. public static void main(String[] args) {int x, y, maxVal;String strNum; strNum = JOptionPane.showInputDialog( "Enter an integer" );x = Integer.parseInt( strNum );strNum = JOptionPane.showInputDialog( "Enter another integer" );y = Integer.parseInt( strNum );maxVal = max( x, y );display( x, y, maxVal ); } public static int max( int num1, int num2 ) {return num1 > num2 ? num1 : num2; } public static void display( int num1, int num2, int max ) {String msg = String.format( "The max of %d and %d is %d.", num1, num2, max );JOptionPane.showMessageDialog(null, msg ); }

8. Testing Methods • HW Problems from the text typically say, “Write a method…” • Thus, you will also need to write a program (main) to test the method with. • For more flexible testing, you can take input from the user and then call the methods • Or, you can hardcode several representative test cases.

9. Testing Methods – From User strNum = JOptionPane.showInputDialog( "Enter an integer" ); x = Integer.parseInt( strNum ); strNum = JOptionPane.showInputDialog( "Enter another int" ); y = Integer.parseInt( strNum ); maxVal = max( x, y ); JOptionPane.showMessageDialog(null, "The max is " + maxVal );

10. Testing Methods – Hardcoded System.out.println( "The max of -2 and -88 is " + max( -2, -88 ) ); System.out.println( "The max of 552 and -2 is " + max( 552, -2 ) ); System.out.println( "The max of 48 and 99251 is " + max( 48, -99251 ) ); System.out.println( "The max of 8 and 8 is " + max( 8, 8 ) ); System.out.println( "The max of 8 and 0 is " + max( 8, 8 ) );

11. Calling Methods Listing 5.1 Testing the max method This program demonstrates calling a method max to return the largest of the int values TestMax

12. animation Calling Methods, cont.

13. animation Trace Method Invocation i is now 5

14. animation Trace Method Invocation j is now 2

15. animation Trace Method Invocation invoke max(i, j)

16. animation Trace Method Invocation invoke max(i, j) Pass the value of i to num1 Pass the value of j to num2

17. animation Trace Method Invocation declare variable result

18. animation Trace Method Invocation (num1 > num2) is true since num1 is 5 and num2 is 2

19. animation Trace Method Invocation result is now 5

20. animation Trace Method Invocation return result, which is 5

21. animation Trace Method Invocation return max(i, j) and assign the return value to k

22. animation Trace Method Invocation Execute the print statement

23. CAUTION A return statement is required for a nonvoid method. The following method is logically correct, but it has a compilation error, because the Java compiler thinks it possible that this method does not return any value. public static int sign(int n) { if (n > 0) return 1; else if (n == 0) return 0; else if (n < 0) return –1; } To fix this problem, delete if (n<0) in the code.

24. Reuse Methods from Other Classes NOTE: One of the benefits of methods is for reuse. The max method can be invoked from any class besides TestMax. If you create a new class Test, you can invoke the max method using ClassName.methodName (e.g., TestMax.max).

25. Call Stacks

26. animation Trace Call Stack i is declared and initialized

27. animation Trace Call Stack j is declared and initialized

28. animation Trace Call Stack Declare k

29. animation Trace Call Stack Invoke max(i, j)

30. animation Trace Call Stack pass the values of i and j to num1 and num2

31. animation Trace Call Stack pass the values of i and j to num1 and num2

32. animation Trace Call Stack (num1 > num2) is true

33. animation Trace Call Stack Assign num1 to result

34. animation Trace Call Stack Return result and assign it to k

35. animation Trace Call Stack Execute print statement

36. Stack view Invoke max Invoke main JBuilder Optional Call Stack in JBuilder Debugger You can see the chain of method invocations in the stack view of the JBuilder debugger.

37. Passing Parameters public static void nPrintln(String message, int n) { for (int i = 0; i < n; i++) System.out.println(message); } Suppose you invoke the method using nPrintln(“Welcome to Java”, 5); What is the output? Suppose you invoke the method using nPrintln(“Computer Science”, 15); What is the output?

38. Pass by Value Listing 5.3 Testing Pass by value This program demonstrates passing values to the methods. TestPassByValue

39. Pass by Value, cont.

40. Overloading Methods Listing 5.3 Overloading the max Method public static double max(double num1, double num2) { if (num1 > num2) return num1; else return num2; } TestMethodOverloading

41. Ambiguous Invocation Sometimes there may be two or more possible matches for an invocation of a method, but the compiler cannot determine the most specific match. This is referred to as ambiguous invocation. Ambiguous invocation is a compilation error.

42. Ambiguous Invocation public class AmbiguousOverloading { public static void main(String[] args) { System.out.println(max(1, 2)); } public static double max(int num1, double num2) { if (num1 > num2) return num1; else return num2; } public static double max(double num1, int num2) { if (num1 > num2) return num1; else return num2; } }

43. Case Study: Computing Taxes with Methods Listing 3.1, “Computing Taxes,” uses if statements to check the filing status and computes the tax based on the filing status. Simplify Listing 3.1 using methods.Each filing status has six brackets. The code for computing taxes is nearly same for each filing status except that each filing status has different bracket ranges. For example, the single filer status has six brackets [0, 6000], (6000, 27950], (27950, 67700], (67700, 141250], (141250, 307050], (307050, ), and the married file jointly status has six brackets [0, 12000], (12000, 46700], (46700, 112850], (112850, 171950], (171950, 307050], (307050, ).

44. Case Study cont. The first bracket of each filing status is taxed at 10%, the second 15%, the third 27%, the fourth 30%, the fifth 35%, and the sixth 38.6%. So you can write a method with the brackets as arguments to compute the tax for the filing status. The signature of the method is: For example, you can invoke computeTax(400000, 6000, 27950, 67700, 141250, 307050) to compute the tax for single filers with $400,000 of taxable income: ComputeTaxWithMethod

45. Scope of Local Variables A local variable: a variable defined inside a method. Scope: the part of the program where the variable can be referenced. The scope of a local variable starts from its declaration and continues to the end of the block that contains the variable. A local variable must be declared before it can be used.

46. Scope of Local Variables, cont. You can declare a local variable with the same name multiple times in different non-nesting blocks in a method, but you cannot declare a local variable twice in nested blocks.

47. Scope of Local Variables, cont. A variable declared in the initial action part of a for loop header has its scope in the entire loop. But a variable declared inside a for loop body has its scope limited in the loop body from its declaration and to the end of the block that contains the variable.

48. Scope of Local Variables, cont.

49. Scope of Local Variables, cont. // Fine with no errors public static void correctMethod() { int x = 1; int y = 1; // i is declared for (int i = 1; i < 10; i++) { x += i; } // i is declared again for (int i = 1; i < 10; i++) { y += i; } }

50. Scope of Local Variables, cont. // With no errors public static void incorrectMethod() { int x = 1; int y = 1; for (int i = 1; i < 10; i++) { int x = 0; x += i; } }